The present invention relates to a hybrid heat exchange apparatus or unit that may be used in different modes as a dry, sensible-heat transfer unit or as an evaporative, latent heat transfer unit, or in a combination mode using both the evaporative and dry cell operation modes, and typically may be used as a condenser to condense gaseous process fluid or as a cooler to cool process fluid in the form of a liquid or non-condensable gas at the temperatures and pressures at which the apparatus is operating.
More particularly, the present invention relates to a hybrid heat exchange apparatus for indirectly transferring heat between a process fluid and ambient air and includes an evaporative heat transfer cell operative in a wet mode or a dry mode, a dry heat transfer cell and a fan for moving air from the ambient atmosphere through the apparatus. The apparatus may be controlled manually or automatically for turning on or off a distributor assembly of cooling liquid in the evaporative heat transfer cell and for controlling air flow through the apparatus. Air is controlled to bypass the dry heat transfer cell to flow through a first air passage or to flow through a second air passage through the dry heat transfer cell and then through the evaporative heat transfer cell. The air flow from the first or second air passages then flows through the evaporative heat transfer cell. The system controller also may allow the air to partially flow through and partially bypass the dry heat transfer cell and then flow through the evaporative heat transfer cell. In another embodiment, ambient air from the first or second air passages may then pass through direct contact evaporative heat exchanger in the evaporative heat transfer cell, while ambient air also flows through a third air passage directly from the ambient atmosphere through an indirect first process fluid coil assembly in the evaporative heat transfer cell. By controlling the flow based on ambient temperature and humidity conditions, the most efficient and economical control of the hybrid heat exchange apparatus can be achieved, while also avoiding potential icing and freezing problems that could otherwise exist in freezing conditions.
Various types of heat exchange apparatus are used in a variety of industries, from simple building air conditioning to industrial processing such as petroleum refining, power plant cooling, and other industries. Typically, in indirect heat exchange systems, a process fluid used any of such or other applications is subject to heating or cooling by passing internally through a coil assembly made of heat conducting material, typically a metal, such as aluminum, copper, galvanized steel or stainless steel. Heat is transferred through the walls of the heat conducting material of the coil assembly to the ambient atmosphere, or in a heat transfer apparatus, to other heat transfer fluid, typically air and/or water flowing externally over the coil assembly where heat is transferred, usually from hot processing fluid internally within the coil assembly to the cooling heat transfer fluid externally of the coil assembly, by which the processing fluid is cooled and the heat transfer fluid is warmed.
In one type of indirect heat transfer, heat is transferred to the atmosphere by dry or sensible heat transfer cells, where there are two fluids: a gas, typically in the form of an air stream externally flowing through a coil assembly of tubes, and a process fluid flowing internally through a coil assembly, which usually have fins to help dissipate the heat. Sensible heat is exchanged as the air stream passes over the coil assembly containing the process fluid stream. As used herein, this type of heat exchanger will be referred to as a “dry heat transfer cell.”
In another type of indirect heat transfer, heat is transferred using indirect evaporative exchange, where there are three fluids: a gas, typically in the form of an air stream, a process fluid flowing internally through a coil assembly of tubes, and an evaporative cooling liquid, typically water, which is distributed over the exterior of the coil assembly through which the process fluid is flowing and which also contacts and mixes with the air or other gas flowing externally through the coil assembly. The process fluid first exchanges sensible heat with the evaporative liquid through indirect heat transfer between the tubes of the coil assembly, since it does not directly contact the evaporative liquid, and then the air stream and the evaporative liquid exchange heat and mass when they contact each other, resulting in more evaporative cooling. Because a cooling liquid is used, sometimes this type of heat exchange is known as a wet type of heat exchange. As used herein, this type of heat exchanger will be referred to as an “indirect evaporative heat transfer cell.”
In other embodiments of an evaporative heat transfer cell, air or other gas and water or other cooling liquid may be passed through direct heat transfer media, called wet deck fill, where the water or other cooling liquid is then distributed as a thin film over the extended fill surface for maximum cooling efficiency. The air and water contact each other directly across the fill surface, whereupon a small portion of the distributed water is evaporated, resulting in direct evaporative cooling of the water, which is usually collected in a sump for recirculation over the wet deck fill and any coil assembly used in the apparatus for indirect heat exchange. As used herein, this type of heat exchanger will be referred to as a “direct contact evaporative heat exchanger.”
As used herein, the term “evaporative heat transfer cell” refers generically to either or both of an “indirect evaporative heat transfer cell,” and to a “direct contact evaporative heat exchanger” unless one type is specified or described more specifically. In evaporative heat transfer cells, the water or other cooling liquid and the air or other gas involved may flow in a concurrent flow (where the liquid and gas flow in the same direction), a countercurrent flow (where the liquid and gas flow in opposite directions, typically the liquid flowing downwardly and the gas flowing upwardly), or a cross-current flow, sometimes referred to as “cross flow,” (where the liquid and gas flow generally transversely with respect to each other, typically the liquid flowing downwardly and the gas flowing across, sideways or generally transverse to the liquid flow direction).
Both dry and evaporative heat transfer cells are commonly used to reject heat as coolers or condensers. Thus, the apparatus of the present invention may be used as a cooler, where the process fluid is a single phase fluid, typically liquid, and often water, although it may be a non-condensable gas at the temperatures and pressures at which the apparatus is operating. The apparatus of the present invention may also be used as a condenser, where the process fluid is a two-phase or a multi-phase fluid that includes a condensable gas, such as ammonia or FREON® refrigerant or other refrigerant in a condenser system at the temperatures and pressures at which the apparatus is operating, typically as part of a refrigeration system where the process fluid is compressed and then evaporated to provide the desired refrigeration. Where the apparatus is used as a condenser, the condensate is collected in one or more condensate receivers or is transferred directly to the associated refrigeration equipment having an expansion valve or evaporator where the refrigeration cycle begins again.
In most climates, evaporative heat transfer cells offer significant process efficiency improvements over dry heat transfer cells. Evaporative coolers reject heat at temperatures approaching the lower ambient wet bulb temperatures, while dry coolers are limited to temperatures approaching the higher ambient dry bulb temperatures. In climates of lower relative humidity, the ambient wet bulb temperature may be 15° F. (−9.4° C.) to 30° F. (−1.1° C.) below the ambient design dry bulb temperature. As a result, in an evaporative cooler, the evaporative liquid stream may reach a temperature significantly lower than the ambient dry bulb temperature, offering the opportunity to increase the efficiency of the cooling process and to lower the overall process energy requirements. Evaporative condensers offer similar possibilities for increased efficiency and lower energy requirements. However, even in view of potentially increased process efficiencies and lower overall process energy requirements, evaporative cooling and evaporative condensing are often not used due to concern about consumption from evaporation of water typically used as the evaporative liquid, and the potential for the water to freeze during cold weather operation.
When designing heat exchange apparatus, both dry heat transfer cells and evaporative heat transfer cells are usually sized to perform their required heat transfer duty under the most challenging thermal conditions, typically expressed as the summer design wet bulb or dry bulb temperature. While it is often critical that the heat exchange apparatus be able to transfer the required amount of heat at these design conditions, the duration of these elevated atmospheric temperatures may account for as little as 1% of the time that the apparatus is operating. The remainder of the time, the apparatus has more capacity than required, resulting in the waste of energy and evaporative liquid.
The present invention, a hybrid heat exchange apparatus, provides more efficient heat transfer, typically cooling by heat rejection, most effectively but not exclusively, at low ambient temperature and low relative humidity, while avoiding freezing and ice formation in the apparatus.